1. Field of the Invention
This invention relates to a method of manufacturing UO.sub.2 -based fuel pellets by recycling irradiated UO.sub.2 -based fuel pellets. More particularly this invention relates to a method of making U.sub.3 O.sub.8 -based powder from irradiated UO.sub.2 -based fuel pellets, mixing the U.sub.3 O.sub.8 -based powder with an additive, pressing and sintering the product to produce the UO.sub.2 -based fuel pellets.
2. Definitions of Terminology
A fresh fuel pellet consists of uranium dioxide (UO.sub.2) or a mixed oxide of uranium dioxide (UO.sub.2) and plutonium dioxide (PuO.sub.2), which mixed oxide is hereinafter referred to as "(U, Pu)O.sub.2 ". In this application, the term "UO.sub.2 -based" refers to both UO.sub.2 - or (U, Pu)O.sub.2 -containing products, and the term "U.sub.3 O.sub.8 -based" refers to both U.sub.3 O.sub.8 - or (U, Pu).sub.3 O.sub.8 -containing products.
3. Description of Related Art
As a UO.sub.2 -based fuel pellet is irradiated in a nuclear reactor, the fissile material in the fuel pellet is depleted and fission products are produced. An irradiated UO.sub.2 -based fuel pellet therefore comprises fissile materials and fission products, of which concentrations are mainly dependent on design burnup specifications and the initial amount of fissile materials. An irradiated fuel pellet discharged from a light water reactor normally has fissile materials of higher than 1% by weight of the irradiated fuel pellet, so it is worthwhile to recycled the fuel pellet for reuse.
According to the literature (G. E. Brand and E. W. Murbach, NAA-SR-11389 (1965)), a UO.sub.2 -based fuel pellet which has been irradiated in a light water reactor can be treated in a so-called AIROX cycle, to refabricate the UO.sub.2 -based fuel pellet. The AIROX cycle comprises the steps of oxidizing irradiated UO.sub.2 -based fuel pellets to U.sub.3 O.sub.8 -based powder, making sinterable UO.sub.2 -based powder from the U.sub.3 O.sub.8 -based powder, mixing the sinterable UO.sub.2 -based powder with enriched fresh UO.sub.2 powder, making granules of the mixed powder, pressing the granules into green pellets, and sintering the green pellets to fabricate UO.sub.2 -based fuel pellets for reuse in a light water reactor.
The art discloses several methods of making sinterable UO.sub.2 -based powder from irradiated UO.sub.2 -based fuel pellets for use in the AIROX cycle. U.S. Pat. No. 3,140,151, discloses a method of making a sinterable UO.sub.2 -based powder comprising oxidizing the irradiated UO.sub.2 -based fuel pellet to U.sub.3 O.sub.8 -based powder in air at a temperature in the range of 300.degree. C. to 500.degree. C., reducing the U.sub.3 O.sub.8 -based powder to UO.sub.2 -based powder at a temperature in the range of 500.degree. C. to 800.degree. C., and repeating the oxidation and reduction steps 3 to 5 times to produce the sinterable UO.sub.2 -based powder. This UO.sub.2 -based powder was able to be sintered to produce UO.sub.2 -based fuel pellets.
A green pellet consisting of U.sub.3 O.sub.8 -based powder which is produced from irradiated UO.sub.2 -based pellets through one round of oxidation can be sintered only up to about 80% theoretical density (TD), and a fuel pellet having such a low density cannot be used in a nuclear reactor, since normal fuel design specifications require the pellet density to be at least about 94% TD. Therefore, oxidation and reduction of the U.sub.3 O.sub.8 -based powder needed to be conducted for many more rounds to enhance its sinterability. In addition, during the oxidation of UO.sub.2 -based pellets or powder to U.sub.3 O.sub.8 -based powder, the irradiated UO.sub.2 -based pellets or powder must be readily pulverized or comminuted to finer powder, since the phase transition of cubic UO.sub.2 to orthorhombic U.sub.3 O.sub.8 causes a volume expansion of about 30%, and thus large stress is generated.
A disadvantage of the prior art is that the oxidation and reduction of UO.sub.2 -based powder needs much time and is hard to control. For example, the oxidation rate of UO.sub.2 -based powder is vety fast, and thus the heat produced from the resultant reaction can increase the temperature of the powder to high temperatures. The UO.sub.2 -based powder so produced is poorly sinterable.